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          <th colspan="3" align="center">Chapter 13.  Berkeley DB Replication </th>
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          <div>
            <h2 class="title"><a id="rep"></a>Chapter 13.  Berkeley DB Replication </h2>
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        </div>
      </div>
      <div class="toc">
        <p>
          <b>Table of Contents</b>
        </p>
        <dl>
          <dt>
            <span class="sect1">
              <a href="rep.html#rep_intro">Replication introduction</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="rep_id.html">Replication environment IDs</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="rep_pri.html">Replication environment priorities</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="rep_app.html">Building replicated applications</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="rep_mgr_meth.html">Replication Manager methods</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="rep_base_meth.html">Base API methods</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="rep_comm.html">Building the communications infrastructure</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="rep_newsite.html">Connecting to a new site</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="group_membership.html">Managing Replication Manager
        group membership</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="group_membership.html#group_mem_add">Adding sites to a replication
            group</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="group_membership.html#group_mem_remove">Removing sites from a
            replication group</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="group_membership.html#group_mem_primordialstartup">Primordial startups</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="group_membership.html#group_mem_upgrade">Upgrading groups</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="rep_partview.html">Replication views</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="rep_filename.html">Managing replication directories and files</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="rep_filename.html#rep_dir">Replication database directory
            considerations</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="rep_filename.html#rep_files">Managing replication internal
            files</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="rep_mgrmulti.html">Running Replication Manager in
        multiple processes</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="rep_mgrmulti.html#idm140526453668224">One replication process and multiple subordinate
            processes</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="rep_mgrmulti.html#idm140526453631248">Persistence of local site network address
            configuration</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="rep_mgrmulti.html#idm140526453609696">Programming considerations</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="rep_mgrmulti.html#idm140526453648912">Handling failure</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="rep_mgrmulti.html#idm140526453608512">Other miscellaneous rules</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="rep_replicate.html">Running replication using the
        db_replicate utility</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="rep_replicate.html#idm140526453598496">One replication process and multiple subordinate processes</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="rep_replicate.html#idm140526453637168">Common use case</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="rep_replicate.html#idm140526453595216">Avoiding rollback</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="rep_replicate.html#idm140526453554320">When to consider an integrated replication application</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="rep_mgr_ack.html">Choosing a Replication Manager acknowledgement policy</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="rep_elect.html">Elections</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="rep_mastersync.html">Synchronizing with a
        master</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="rep_mastersync.html#rep_delay_sync">Delaying client synchronization</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="rep_mastersync.html#rep_c2c_sync">Client-to-client synchronization</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="rep_mastersync.html#idm140526453549456">Blocked client operations</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="rep_mastersync.html#idm140526453475616">Clients too far out-of-date to synchronize</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="rep_init.html">Initializing a new site</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="rep_bulk.html">Bulk transfer</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="rep_trans.html">Transactional guarantees</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="rep_lease.html">Master leases</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="rep_lease.html#masterlease_change_groupsize">Changing group
            size</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="rep_ryw.html">Read your writes consistency</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="rep_ryw.html#gettoken">Getting a token</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="rep_ryw.html#tokenhandling">Token handling</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="rep_ryw.html#usingtoken">Using a token to check or wait for a transaction</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="rep_clock_skew.html">Clock skew</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="comm_repsites.html">Communicating between Replication Manager Sites</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="comm_repsites.html#conf_writeforw">Configuring for Write Forwarding</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="comm_repsites.html#repmgr_channels">Using Replication Manager message channels</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="rep_twosite.html">Special considerations for two-site replication groups</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="rep_twosite.html#twosite_strict">Two-site strict configuration</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="rep_twosite.html#twosite_prefmas">Preferred master mode</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="rep_twosite.html#twosite_other">Other alternatives</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="rep_partition.html">Network partitions</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="rep_faq.html">Replication FAQ</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="rep_ex.html">Ex_rep: a replication example</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="rep_ex_comm.html">Ex_rep_base: a TCP/IP based communication infrastructure</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="rep_ex_rq.html">Ex_rep_base: putting it all
        together</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="rep_ex_chan.html">Ex_rep_chan: a Replication Manager channel example</a>
            </span>
          </dt>
        </dl>
      </div>
      <div class="sect1" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h2 class="title" style="clear: both"><a id="rep_intro"></a>Replication introduction</h2>
            </div>
          </div>
        </div>
        <p>
        Berkeley DB includes support for building highly available
        applications based on replication. Berkeley DB replication
        groups consist of some number of independently configured
        database environments. There is a single
        <span class="emphasis"><em>master</em></span> database environment and one
        or more <span class="emphasis"><em>client</em></span> database environments.
        Master environments support both database reads and writes;
        client environments support only database reads. If the master
        environment fails, applications may upgrade a client to be the
        new master. The database environments might be on separate
        computers, on separate hardware partitions in a non-uniform
        memory access (NUMA) system, or on separate disks in a single
        server. As always with Berkeley DB environments, any number of
        concurrent processes or threads may access a database
        environment. In the case of a master environment, any number
        of threads of control may read and write the environment, and
        in the case of a client environment, any number of threads of
        control may read the environment.
    </p>
        <p>
        Applications may be written to provide various degrees of
        consistency between the master and clients. The system can be
        run synchronously such that replicas are guaranteed to be
        up-to-date with all committed transactions, but doing so may
        incur a significant performance penalty. Higher performance
        solutions sacrifice total consistency, allowing the clients to
        be out of date for an application-controlled amount of
        time.
    </p>
        <p> 
        There are three ways to build replicated applications. The
        simpler way is to use the Berkeley DB Replication Manager. The
        Replication Manager provides a standard communications
        infrastructure, and it creates and manages the background
        threads needed for processing replication messages. 
    </p>
        <p>
        The Replication Manager implementation is based on TCP/IP
        sockets, and uses POSIX 1003.1 style networking and thread
        support. (On Windows systems, it uses standard Windows thread
        support.) As a result, it is not as portable as the rest of
        the Berkeley DB library itself.
    </p>
        <p>
        For applications with simple data and transaction models,
        Replication Manager provides automatic write forwarding as 
        a configurable option. Use of this option enables some
        write operations to be performed on a client environment.
        For more information, see
        <a class="xref" href="comm_repsites.html" title="Communicating between Replication Manager Sites">Communicating between Replication Manager Sites</a>.                
    </p>
        <p>
         If for some reason using Replication Manager or write forwarding
         does not meet your application's technical requirements, you will
         have to use the lower-level replication "Base
         APIs". This approach affords more flexibility, but requires
         the application to provide some critical components:
    </p>
        <div class="orderedlist">
          <ol type="1">
            <li>
            A communication infrastructure. Applications may use
            whatever wire protocol is appropriate for their
            application (for example, RPC, TCP/IP, UDP, VI or
            message-passing over the backplane).
        </li>
            <li>
            The application is responsible for naming. Berkeley
            DB refers to the members of a replication group using an
            application-provided ID, and applications must map that ID
            to a particular database environment or communication
            channel.
        </li>
            <li>
            The application is responsible for monitoring the
            status of the master and clients, and identifying any
            unavailable database environments.
        </li>
            <li>
            The application must provide whatever security
            policies are needed. For example, the application may
            choose to encrypt data, use a secure socket layer, or do
            nothing at all. The level of security is left to the sole
            discretion of the application.
        </li>
          </ol>
        </div>
        <p>
        (Note that Replication Manager does not provide wire
        security for replication messages.)
    </p>
        <p>
        The following pages present various programming
        considerations, many of which are directly relevant only for
        Base API applications. However, even when using Replication
        Manager it is important to understand the concepts.
    </p>
        <p>
        Finally, the Berkeley DB replication implementation has one
        other additional feature to increase application reliability.
        Replication in Berkeley DB is implemented to perform database
        updates using a different code path than the standard ones.
        This means operations that manage to crash the replication
        master due to a software bug will not necessarily also crash
        replication clients.
    </p>
        <p>
        For more information on the Replication Manager operations,
        see the <a href="../api_reference/C/rep.html#replist" class="olink">Replication 
        and Related Methods</a> section in the
        <em class="citetitle">Berkeley DB C API Reference Guide.</em>
    </p>
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